This is not a way to get Mac OSx server for free, just how I got it, but by accident.

Tiger to Leopard
So I bought two brand new Macbook Pros this time two years ago, both came pre installed with Tiger and I had the option to upgrade to Leopard as it was just released. I filled in the paperwork and for $9.99 x 2 I got 2 shiny new Leopard upgrade disks from Apple.
The upgrade for both macs went as easy as you can imagine and after a brief update session from Apple I then had the latest and greatest version of Leopard running.
All was well until some time last month. My Macbook started to run unusually slow, so first things first I run the cron jobs – MacJanitor – Free and essential for Macbook users – Mac
Janitor…
hmmm….still no change, time to crank up Disk Utility (Applications>Utilities>Disk Utility.app). So I start up by verifying the disk permission, then repairing them, sure there are a few issues from the upgrade from Tiger to Leopard, but nothing worth worrying about. So next step is the Verify Disk. Yikes! Disk error found….no panic, I have Time Machine running on a 1TB drive, worst case scenario is a new hard drive and install from a Time Machine image.
So you fix a Disk error it is best to boot using the OSx disk. If you don’t know how to do this, then read this article from Apple. (http://support.apple.com/kb/HT1782 ). Normally that would work, but I made a mistake. I popped in the original Tiger disk. The error was found, but could not be fixed, DUH!. So I can’t find the Leopard upgrade disk. I went to Apple.com, order a call back for 15 minutes and yep, 15 minutes later I am talking to Josh the Mac Genius. This is another reason for being a Mac customer. So I get Josh up to speed and yes, we need to find the Leopard upgrade disk. I can’t, so he orders me a new one, free of charge. Call ends. Five minutes later my co-worker arrives with her Mac and can you believe it she has the Leopard upgrade disk. I pop it into my Mac, book to the disk, choose Disk Utilities, Repair the disk, disk fixed, reboot to my internal hard drive, check with Disk Utility and yep, we are good to go. My Mac is running full steam again.
A few days later I get a package from Apple. I assume it is my new Leopard upgrade disk. Nope, by accident it is a Mac OSx Leopard Server Disk (x2). – Time to build a Hackintosh Mac OSx Server! Whoo Hoo!